Transplantation specificity and protective immunity occur in both adaptive and innate branches of the vertebrate immune system. However, the mechanisms which underlie the specificity, development, and self-tolerance of innate recognition strategies are not well understood. Botryllus schlosseri is born a chordate tadpole larva and later metamorphoses to an invertebrate adult, and represents the evolutionary transition between invertebrates and vertebrates. B. schlosseri also undergoes a naturally occurring transplantation reaction when two individuals come into contact which will result in either fusion of extracorporeal blood vessels to form natural chimeras, or an active, blood-based, cell-based immunity which blocks vascular fusion. This reaction is controlled by a single, highly polymorphic locus called the fuhc. In the last round of funding we isolated the fuhc locus by positional cloning and identified a highly polymorphic single gene which predicts histocompatibility outcomes in wild-type individuals. In addition, we also identified a putative group of receptors linked to the fuhc, the fester family, which we have shown are functionally involved in allorecognition events. The fuhc is one of the most polymorphic genes ever described, with normal populations having tens to hundreds of alleles. However, the ascidians do not have somatic diversification processes, such as recombination or somatic hypermutation found in vertebrate immunity. Thus an innate effector system demonstrates exquisite discriminatory ability, and our long-range goal is to dissect the cellular molecular mechanisms responsible. PUBLIC HEALTH RELEVANCE: Our proposed studies will continue to provide novel insights into the mechanisms an individual uses to discriminate between self and non-self. This reaction is the basis of immunity, however the mechanisms which are responsible are not well known. Understanding these mechanisms has major significance, from preventing a rejection reaction after transplantation, to understanding what is breaking down during autoimmune disease, when the immune system attacks itself. We are studying this process in a species which represents the transition between vertebrates and invertebrates, called a tunicate. The species we study, called Botryllus, undergoes a natural transplantation reaction which in many ways resembles how humans accept or reject bone marrow transplants. This organism also offers unique ways to study these reactions- the reaction is fast, occurring in less than one day, and occurs outside the body, allowing us to visualize and manipulate the cells involved. This is an ancestral species which demonstrates a basal form of immunity, properties of which are the building blocks of evolution. Studying the biology underlying self/non-self recognition in a simpler, ancestral organism will provide opportunities and insights not available in other species.